this makes no sense for shadowpriests... if you are min maxing then it counts as about 1.6 dmg with imp spirit (which any high end raid will have since you can do it with an alt now)and if you have the 30% regen talent (which almost every priest has) then it also counts as regen too(even though regen is irrelevant to any priest with high end gear since they will almost never run out of mana)

you should really make it a 10 value for shadowpriests (for non shadowpriests its much more debateable)